Electric eddy current furnace



Dec. 8, 1953 DREYFUS 2,662,161

ELECTRIC EDDY CURRENT FURNACE Filed April 5, 1950 //1 en for Patented Dec. 8, 1953 UNITED STATES Z,662,i i

PATENT OFFICE ELECTRIC EDDY CURRENT FURNACE Ludwig Dreyfus, Vasteras, Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a Swedish corporation When electric eddy current furnaces, generally operated by frequency current, are employed for heating worl: pieces, for instance for forging, it is important to adapt the furnace for work pieces of different dimensions in a simple and heat economi ing manner. For a certain density of the primary ampercturns, corresponding to normal heating speed, the power losses of the furnace will be substantially proportional to the inner surface of the current-carrying winding, while the useful power becomes proportional to the outer surface of the heated work piec For this reason, it is important for avoiding unneccssary losses, that the work piece fills out the active portion of the furnace winding as far as possible. If the cross section of the work piece varies between wide limits, there is as a rule no other way possible then the use of different furnaces work pieces of essentially different cross sections. If, on the other hand, the length of the work piece r of the heated portion thereof varies, the length of the current-carrying winding may be varied in a certain furnace.

The problem then arises how the condensers necessary for compensating the reactive power demand of the winding should be connected. The most near-at-hand connection, w ich also probably has been in practice, seems to be to connect a variable number of condensers in parallel to the furnace winding, as it is usually practiced when reactance varies during the operation of the furnace. The consequence hereof is, however, that the smallest possible winding portion connected in circuit corresponds to the largest number of condensers and vice versa, whence the whole condenser battery must be made considerably larger than that corresponding to the total reactance of the furnace winding. It is not possible to connect the condensers simply in series like the winding portions, as this would make the change of connections too complicated.

In order to limit the total reactive power of the condenser battery and to facilitate at the same time a compensation of the reactive power demand at any active len th of the winding by the smallest possible number of different connections, the present invention provides a parallel connection of two or more portions of the furnace winding through an interphase transformer. As the case may be, small portions of the furnace winding be connected in series with those connected in parallel for still increasing the adaptability to different lengths of work pieces, the condensers being then connected in a particular manner to be described hereinafter for facilitating the change of connections.

In the accompanying drawing, 1 shows an eddy current furnace embodying one form of the invention in a longitudinal section with the source of current, interphase transformer, condensers and switches only diagrammatically shown. Figs. 2 and 3 show two other forms in which also the furnace winding is shown in a purely diagrammatic Way.

In Fig. l, 1 and 2 designate two portions of a furnace winding which surrounds an insulating tube 29 adapted to receive the work piece to heated. The conductors of the winding are shown as tubular for being traversed by a cooling medium. The two portions have different numbers of winding turns, the portion 2 having twice as many turns as the portion i. The inner ends of the two winding portions are connected through an interphase transformer c to one terminal of a high frequency generator while the outer ends of the winding portions are connected through switch means to and 6b to the other terminal of the said high frequency generator. Another switch '3 may serve to short-circuit the interphase transformer 3. Each winding portion is connected, through switch means so, to re spectively, in parallel to a condenser 23, Q respectively of a capacity corresponding to the reactance of said winding portion.

If, as aforesaid, the winding portion .2 has twice the length of the portion i, it becomes possible, by connecting the portion i alone, the portion alone, or both portions together in circuit, to vary the active furnace length in the proportions 1:213. In connecting only one of the portions in circuit, the interphase transformer is short-circuited by means of the switch i. One half of the said transformer is still connected. between the generator and the furnace winding, but it cannot take up any voltage, as the whole transformer is short-circuited. Of the condensers, only that is left in circuit which belongs to the temporarily active winding portion. If one of these portions, as i, is substantially shorter than the other one, as shown, some winding turns iii may be mounted outside the former and connected in series therewith. As these turns are connected between one generator terminal and the point from which the furnace winding 5 and the compensating condenser 8 are connected in parallel, they can only carry the generator cur rent, which forms a small part of the total furnace current, and therefore they cause no appreciable heating action, but as they have substantially the same voltage per winding turn as the active winding, they absorb an appropriate portion of the generator voltage, which as a rule should not be varied, as several furnaces may be connected in parallel to a common generator.

In the first case, when only the winding portion I is to be active, the interphase transformer is short-circuited by closing the switch 1 and the switches a and 5b are closed. The current then flows in two paths, one through the left hand part of coil 3, through winding portion I and through coil II], the other through condenser B and coil I0, and the interphase transformer takes: up no voltage, as it is short-circuited. In the second case, when only winding .2 is to be active, the interphase transformer is still short-Mr cuited, while the switches ca andrtb are closed and the switches 57) and 5a opened. The operation is analogous to that just described forthe first case, except that the winding portion 2 with its condenser 9 will be in circuit insteadof the winding portion I with its condenser 8. As the winding portion 2 has twice the length of the portion I, the active furnace length will nowbe twice that in the first case. In the third case, when portions I and 2 are both to be active, the switch I is opened and the interphase transformer 3 is thus active, and further the switch 6?) is still closed and the switch 51) preferably left open. The generator then delivers current directly only to the Winding portion 2 but through the interphase transformer also to the portion I. The switch 5a is still closed, whence the condenser 8 delivers the majority of the reactive power to the winding portion I, but, as the generator 4 is now not connected to the winding portion I, the condenser 8 is, according to the usual aspect, connected in series with that portion instead of in parallel therewith, as in the first case. Theoretically, it may also be possible to close the switch 51), whereby both winding portions are connected in parallel to the generator and to their respective condensers, although this would load the interphase transformer rather excessively. When both winding portions are active, the active furnace length is three times that of the first case.

When both winding portions are in circuit, the generator is preferably connected only to the larger winding portion by closing-the switch 65 and leaving the switch 51) open. The interphase transformer, which is then active, forces the same current through both winding portions, and the voltages are then determined by' the reactances of the portions. Possible voltage dif-- ferences are absorbed by the interphase transformer, but in order to make them as small as possible, the reactances of the condensers should be adapted to those of the winding portions as closely as possible.

In many cases it is desirable, paying regard to thelocal working conditions, to heat only one end of a work piece and to introduce the pieces only at one end portion of the furnace. Especially in such cases it has been found advisable to divide the furnace into two equal portions connected in parallel and to vary hereby the active length only in the proportion 41:2,. In such a case, as also in combination with a subdivision according to Fig. l, the possibility of .a variation in length may be increased by connecting, in 1 series with one or both of the parallel-connectedmain portions, a winding portion the condenser of which is, by means of a two-way switch, seriesconnected with the parallel condenser of the main portion at the same time as the two winding portions are series-connected. In this manner, a change of connections between these two active lengths of the furnace may also be effected by a single manual operation.

An example of the last-mentioned way of connection is diagrammatically shown inFig. 2. The two main winding portions connected in parallel are here designated by I I and I2, the interphase transformer connected between them by I3 and the short-circuiting switch therefore by H. The generator is designated by I4, the main switches by I5, I6 and the main condensers by I8, IS. The winding portions connected in series with theportions I I, I2 are designated by 2|, 22, their condensers by 23, 24 and the switches for their disconnection by 25, 26.

If in Fig. 2 the switch 25 is put in its left po sition and the switch 26 in its right position and the switch I! is opened, the whole furnace will be in circuit, corresponding for instance to an active length of seven units, if it is assumed that the lengths of the portions 2|, II, I2, 22 corre* spond to 2, 2, 2 and I unit respectively. If now the switch 25 is put to the right, the winding 2 portion 2! and its parallel condenser aresimultaneously cut out, resulting in'an active furnace length of for instance five units; If further the interphase transformer I3 is short-circuited by means of the switch l1 and the switch 25'put in its middle position, only the windingportions'to the right I2 and 22 will remain in circuit, cor

responding for instance to"an active furnace length of three units.

furnace lengths.

If the two main portions of the furnace windings have essentially equal numbers of turns, as in Fig. 2, their condensers may be united inone single'battery, connected between the Zero pointof the interphase transformer and a busbar common to the outer winding terminals, as shown in Fig. 3. The main portions of the furnace winding are here designated by 3|, 32, the interphase transformer by 33,the generator by 34, the common condenser battery by 35, the additional winding portions by'3'5, 31, and their parallel condensers by 38, 39. About half of the common condenser battery should however be capable of being cut out when only one main portion of the-- winding shall be in circuit. A connection of this kind affords particularly the advantage, that if made in the main battery, so that a separate adjustment of the series condensers 38, 33 becomes unnecessary.

Fig. 3 shows, with respect to Fig. 2, also the modification independent of that now described, that the generator34- is directly connected to the furnace winding instead of to the condenser terminals, the generator being capable of being connected to different taps of the furnace wind ing, whereby still a possibility of meeting (hf-'- --ferent operating'conditions is afforded. Thisis Finally, by putting theswitch 26 to the left, the winding portion 22 and= its parallel condenser may be out out, wherebythe active length is still further reduced, sayto" two units. The lengths of the separate windings portions may of course be differently chosen sons to give different proportions between the active 5 especially important in feeding several furnaces in parallel from a common generator, when it is desired to vary the operating voltages of the different furnaces independently of each other.

In practice, each furnace is frequently adapted to hold several work pieces situated side by side and then often with an oblong cross-section so as to make the work pieces lie close to the winding at least on two or three sides.

I claim as my invention:

1. Electric eddy current furnace adapted to heat work pieces of varying length, comprising a substantially cylindrical winding surrounding a space for receiving work pieces and divided into portions connected in parallel through an interphase transformer connecting one end of one winding portion to one end of another one, condenser means connected in parallel to said winding portions and a source of alternating current adapted to feed said winding portions through said interphase transformer.

2. Electric eddy current furnace adapted to heat work pieces of varying length, comprising a substantially cylindrical winding surrounding a space for receiving work pieces and divided into portions connected in parallel through an interphase transformer connecting one end of one winding portion to one end of another one, some of said portions being further divided into seriesconnected parts, condensers capable of being connected in parallel with each of said parts, and a source of alternating current adapted to feed said winding portions through said interphase transformer.

3. Electric eddy current furnace adapted to heat work pieces of varying length, comprising a substantially cylindrical winding surrounding a space for receiving work pieces and divided into portions connected in parallel through an interphase transformer connecting one end of one winding portion to one end of another one, some of said portions being further divided into seriesconnected parts, condensers for compensating the individual reactances of said series-connected parts, switch means for cutting in and out certain of said parts and condensers, and a source of alternating current adapted to feed said winding portions, through said interphase transformer.

4. Electric eddy current furnace adapted to heat work pieces of varying length, comprising a substantially cylindrical winding surrounding a space for receiving work pieces and divided into portions connected in parallel through an interphase transformer, condenser means connected between the midpint of said interphase transformer and a conductor common to the opposite terminals of the parallel-connected winding portions, and a source of alternating current adapted to feed said winding portions.

5. Electric eddy current furnace adapted to heat work pieces of varying length, comprising a substantially cylindrical Winding surrounding a space for receiving work pieces and divided into portions, an interphase transformer having its ends connected to one end of each of said portions respectively, a source of alternating current having one output terminal connected to an intermediate point of said interphase transformer, connections from the other output terminal of said source to the other ends of said portions, connections between such other ends and the first output terminal, condensers in said last connections, and at least one switch in one of said connections.

LUDWIG- DREYFUS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,623,839 I-Iundt Apr. 5, 1927 1,698,581 Campbell et a1. Jan. 8, 1929 1,715,215 Whish May 28, 1929 1,791,934 Northrup Feb. 10, 1931 1,813,488 Field July 7, 1931 1,849,309 Northrup Mar. 15, 1932 1,943,802 Northrup Jan. 16, 1934 1,992,809 Blume Feb. 26, 1935 2,052,649 Patterson Sept. 1, 1936 2,482,493 King Sept. 20, 1949 2,551,757 Mittelmann May 8, 1951 

